1. Field of the Invention
The present invention relates to a six-stroke cycle engine that includes a scavenging stroke and executes an intake stroke, a compression stroke, an expansion stroke, an exhaust stroke, a scavenging intake stroke, and a scavenging exhaust stroke, in this order.
2. Description of the Related Art
As an engine capable of lowering the cylinder temperature, there is conventionally known a six-stroke cycle engine including a scavenging stroke (scavenging intake stroke and scavenging exhaust stroke) after an exhaust stroke. “Scavenging” of the scavenging stroke means an operation of replacing the gas in the cylinder. In this specification, the scavenging intake stroke indicates a stroke of supplying air into the cylinder to replace the gas in the cylinder. The scavenging exhaust stroke indicates a stroke of discharging air from the cylinder to replace the gas in the cylinder.
In a conventional six-stroke cycle engine, when fresh air is supplied into the cylinder in the scavenging intake stroke, the wall surface of the cylinder is cooled, and the temperature of the cylinder is lowered. When the temperature of the cylinder is lowered, the compression ratio can be made high, and the fuel efficiency or output can be improved. In addition, the ignition timing can be advanced, and the output at the time of high speed rotation can be improved. Furthermore, since the fresh air remains in the cylinder after the scavenging exhaust stroke, the intake volumetric efficiency can be improved in the next intake stroke.
In the six-stroke cycle engine of this type, however, since fresh air is discharged to the exhaust passage in the scavenging exhaust stroke, the following problems arise. More specifically, an oxygen density detected by an O2 sensor provided in the exhaust passage greatly increases, and correct execution of air-fuel ratio control becomes impossible. In addition, since a large quantity of oxygen is supplied to the catalyst provided in the exhaust passage, oxidation reaction excessively occurs in the catalyst. Hence, the temperature of the catalyst abruptly rises, and NOx cannot be reduced.
To solve these problems, an arrangement for returning air in the cylinder to an intake passage in the scavenging exhaust stroke, as described in, for example, Japanese Patent Laid-Open No. 2010-209683, may be used.
A six-stroke cycle engine disclosed in Japanese Patent Laid-Open No. 2010-209683 includes a scavenging passage that communicates a combustion chamber with an intake passage, and a scavenging valve that opens and closes the scavenging passage. In this six-stroke cycle engine, an intake valve opens to supply fresh air into the cylinder in the scavenging intake stroke, and the scavenging valve opens to discharge the air in the cylinder to the intake passage in the scavenging exhaust stroke. That is, in this six-stroke cycle engine, since no fresh air is discharged to the exhaust passage in the scavenging exhaust stroke, the above-described problems do not arise.